As a new technique, cloud computing has a rapid development in recent years. However, the security problems have caused great influences to the development and popularization of cloud computing, the importance and urgency has not to be ignored. This paper introduces cloud computing and security situation, studies the main security problems of cloud computing, and comes up with a cloud computing security framework which can effectively solve these security problems, and points out that only to solve the security problems, cloud computing can unceasingly expanded, and the application will be more and more widely.